The invention relates to a process for the production of a valve metal oxide powder, in particular an Nb2O5 or Ta2O5 powder, and to valve metal oxide powders obtainable in this manner which exhibit a spherical morphology, an average particle size of 10 to 80 μm and an elevated BET surface area.
Valve metals, especially those from subgroups 4 to 6 of the periodic system of elements and among these in particular tantalum and niobium, and the alloys thereof, have many and varied applications. Valve metals are generally produced by reduction of suitable valve metal compounds, in particular by reduction of valve metal oxides.
Valve metal oxide powders are, however, of interest not only as starting materials for the production of corresponding metal powders, but also for numerous further applications. For example, niobium and tantalum oxides with elevated specific surface areas are used in the production of mixed metal oxide materials which have applications, for example, as catalysts and/or functional ceramics.
If, when producing such metal oxide materials, it is desired to achieve not only good intermixing of tantalum oxide and/or niobium oxide with the further reactants, such as for example potassium carbonate or molybdenum trioxide, but also performance of the reaction at the lowest possible temperature, a spherical morphology of the valve metal oxides in conjunction with an elevated specific surface area are advantageous. In “Catalysis Today 78 (2003) 47-64”, M. Ziolek provides a review of niobium-containing catalysts. The most important class of compounds is stated to be niobium oxides which should if possible exhibit an elevated specific surface area.
Processes for the production of niobium and tantalum oxides with elevated specific surface area have already been described in the literature. However, the oxides produced by these processes do not generally exhibit a spherical morphology or they are nanoscale valve metal oxide powders.
DE 4 214 724 C2 accordingly describes the production of fine ceramic powders of a narrow grain size distribution in a gas phase reaction. By reacting niobium or tantalum pentachloride with oxygen, it is possible in this manner to produce niobium and tantalum pentoxides which, according to the Example, exhibit a specific surface area of 42 m2/g. However, due to the performance of the reaction in the gas phase and the liberation of gaseous chlorine, this process is highly complex. The Nb2O5 produced according to the Example moreover contains a total of 700 ppm of metallic impurities.
In “Materials Transactions, vol. 42, no. 8 (2001), 1623-1628”, T. Tsuzuki and P. G. McCormick describe a mechanochemical synthesis for niobium pentoxide nanoparticles. In this synthesis, solid niobium pentachloride is reacted with solid magnesium oxide or sodium carbonate to produce Nb2O5 with an elevated specific surface area of 43.3 to 196 m2/g. However, solid-phase reactions proceed only very slowly. Reaction times of several hours are described. A further disadvantage of this method is that, due to the process, the resultant products are severely contaminated with sodium. Niobium pentoxides contaminated in this manner have a tendency when heat treated (T>550° C.) to form Na2Nb4O11 phases.
In “Topics in Catalysis, vol. 19, no. 2, 2002, 171-177”, J. N. Kondo, Y. Takahara, B. Lee, D. Lu and K. Domen describe processes for the production of mesoporous tantalum oxides. Using the so-called NST (neutral surfactant template) method, tantalum(V) chloride is hydrolysed by means of the moisture present in ambient air by addition of the chelating ligand poly(alkylene oxide) block copolymer Pluronic P-123 (BASF). The resultant Ta2O5 exhibits a very high specific surface area. Disadvantages of this process are not only the long reaction time of at least 6 days but also the evolution of gaseous HCl. Ta2O5 with an elevated specific surface area of 330 to 410 m2/g is also obtained by the so-called LAT (ligand-assisted templating) method. According to this method, tantalum(V) ethoxide is hydrolysed with addition of octadecylamine. However, the resultant product is neither thermally nor mechanically stable and is thus not usable for large scale industrial applications or for further processing. In addition, the tantalum(V) ethoxide used is very costly.
Nanoscale Nb2O5 powders with elevated specific surface areas may also be prepared according to C. Feldmann and H.-O. Jungk (Angew. Chem. 2001, 113, no. 2, 372-374) by hydrolysis of niobium ethoxide in diethylene glycol. Niobium pentoxides prepared in this manner exhibit a specific Brunauer-Emmett-Teller (BET) surface area of about 100 m2/g. Disadvantages of this process are that the tantalum(V) ethoxide used is very costly and only nanoscale oxide particles can be obtained.
Niobium pentoxide with an elevated specific surface area of 232 m2/g may also be prepared according to H. Kominami, K. Oki, M. Kohno, S. Onoue, Y. Kera and B. Ohtani (Journal of Materials Chemistry 2002, 11(2), 604-609) by hydrolysis of niobium butoxide in toluene. Disadvantages of this process are both the possible environmental impact associated with the use of toluene as solvent and the high price of the niobium butoxide used.
DE 103 07 716 A1 discloses that spherical niobium and tantalum oxides may be produced by precipitation of heptafluorotantalic acid (H2TaF7) or heptafluoroniobic acid (H2NbF7) or mixtures thereof from a hydrofluoric solution by means of bases, in particular ammonia (NH3). This yields tantalic acid Ta(OH)5 or niobic acid Nb(OH)5 or mixtures thereof, which may then be converted into the corresponding oxide by heat treatment or calcination as it is known. These oxides, however, exhibit low specific surface areas of 0.41 to 0.58 cm2/g.
WO 97/13724 A1 discloses a process for the production of valve metal oxides by precipitating H2TaF7 or H2NbOF5 by means of ammonia. Precipitation is performed in at least two reaction vessels connected in series, wherein temperature, pH and residence time are separately adjusted in each reaction vessel. In this manner, it is possible purposefully to adjust the specific surface areas and densities of the valve metal oxides produced. Valve metal oxides with an elevated surface area and low density and valve metal oxides with a small surface area and high density are described. According to WO 97/13724 A1, valve metal oxides with an elevated surface area are, however, taken to mean those valve metal oxides which exhibit a BET surface area of greater than 2 m2/g (Nb2O5) or of greater than 3 m2/g (Ta2O5). The maximum BET surface area value stated for tantalum pentoxide particles is 11 m2/g. The maximum BET surface area obtained in the Examples is 6.7 m2/g (Example 6). Scanning electron micrographs of valve metal oxides with an elevated surface area show that these products exhibit irregular morphologies (FIGS. 3A to 3D and FIGS. 5A to 5D). Spherical valve metal oxide powders with an elevated BET surface area thus cannot be obtained according to WO 97/13724 A1. A further disadvantage of the procedure according to WO 97/13724 A1 is that, because the reaction is performed in at least two reaction vessels in which the essential process parameters must in each case be separately adjusted, it is associated with greater complexity of the control system.